The present invention relates to medical electrical leads generally, and more particularly to implantable defibrillation electrodes and leads.
Early concepts of implantable defibrillators, such as disclosed in Reissue U.S. Pat. No. 27,652 by Mirowski et. al., envisioned an electrode system employing a ventricular endocardial electrode and a plate electrode mounted to the heart or implanted subcutaneously. However, commercially available implantable defibrillators employ epicardial electrodes, the installation of which requires a thoracotomy.
It is generally understood that it would be desirable to produce an implantable defibrillation system which entirely avoids the necessity of a thoracotomy, and there has been substantial work directed towards development of such systems, as disclosed in U.S. Pat. No. 4,727,877 issued to Kallok, U.S. Pat. No. 4,708,145 issued to Tacker et. al., and as disclosed in allowed U.S. application Ser. No. 07/284,955 filed Dec. 15, 1988 by Bardy, for an "Endocardial Defibrillation Electrode System", incorporated herein by reference in its entirety. Electrode systems of this type are presently in clinical evaluation, pending FDA approval.
Other endocardial defibrillation electrodes are disclosed in U.S. Pat. No. 4,481,953 issued to Gold et. al., U.S. Pat. No. 4,161,952 issued to Kinney et. al., U.S. Pat. No. 4,934,049 issued to Kiekhafer et. al. and in U.S. Pat. application Ser. No. 07/479,928, filed Feb. 14, 1990 by Holleman et. al., for an "Implantable Electrode and Method for Fabrication", all of which are incorporated herein by reference in their entireties. The Kinney, Gold and Kiekhafer patents and the Holleman et. al. application all disclose endocardial defibrillation leads employing defibrillation electrodes fabricated from elongated coils of biocompatible metal, mounted exposed to the exterior of the defibrillation lead, for location in the right ventricle and other locations within the heart. U.S. Pat. No. 4,641,656. issued to Smits and the above cited Bardy application both disclose a variety of endocardial defibrillation electrodes intended for use in the atrium, ventricle and coronary sinus, all of which employ electrodes taking the form of elongated coils of conductive biocompatible metals.
The endocardial leads set forth in the above cited references are generally employed with one or more additional endocardial or subcutaneous electrodes. In general, there has been a trend toward lead systems employing three or more such electrodes in order to reduce defibrillation thresholds to an acceptable level. In the Tacker and Kallok references, lead systems which employ three or more electrodes, sequentially paired with one another are discussed. In the Bardy application and the Smits patent, lead systems in which three or more electrodes are used simultaneously to deliver a defibrillation pulse are disclosed.
The subcutaneous leads employed in the systems as discussed above may be fabricated using metal mesh electrodes, as disclosed in U.S. Pat. No. 4,765,341, issued to Mower et. al., coiled metal wire electrodes as disclosed in U.S. Pat. No. 4,817,634, issued to Holleman et. al. or may be the metal enclosure of the defibrillator as disclosed in the above-cited Kallok patent.
A variety of pulse wave forms and polarities have been suggested. Monophasic capacitive discharge pulses are disclosed in the above cited Mirowski reissue patent. Biphasic pulses are disclosed in U.S. Pat. No. 4,953,551, issued to Mehra et. al. Damped sinusoidal pulses are disclosed in U.S. Pat. No. 4,834,100, issued to Charms.
A return to lead systems employing only two electrodes is suggested in U.S. Pat. No. 4,922,927, issued to Fine et. al. This patent proposes the use of an electrode system as in the above-cited Mirowski reissue Patent, using a right ventricular electrode and a subcutaneous electrode, which may correspond to prior art subcutaneous electrodes or may be the metal enclosure of the defibrillator. The right ventricular electrode carries an elongated coil electrode fabricated of a copper-zirconium alloy coated with iridium oxide. The use of biphasic pulses in such a two electrode system is also recommended. However, no recommendation is given as to the preferred polarity for the leading phase of the biphasic pulse. The Fine patent states that defibrillation thresholds as low as 7-10 joules may be achieved with such an endocardial lead in conjunction with a subcutaneous electrode, apparently implanted in proximity to the ventricles rather than pectorally.